Local deepening of the thalweg (colk formation) on one or both sides of constrictions in tidal channels is described in Brisbane Water (New South Wales), Port Phillip (Victoria), and Otago Harbour (New Zealand). Since channel deepening at the site of the restriction has already been described by Suess (1906) and the term “scape colk” introduced to designate it, it is suggested that the excavations in the expanded channel described in this paper should be termed “scour colks”. The mechanism of colk formation has important applications in harbour engineering, and further studies are desirable preferably by means of hydraulic models.